Introduction to Plasma Instabilities

Abstract

Particles in space plasmas generally have a large mean free distance between interparticle collisions. For example, the mean free distance of solar wind electrons is known to be approximately 1 A. U. near the earth. In terms of the mean free time, this means that such a particle will collide only once every few days. Furthermore, if we note that the energy equipartition time is longer than the interparticle collision time by the mass ratio of proton and electron, we can expect that the plasmas in space, either interplanetary or magnetospheric, are not in thermodynamical equilibrium. However, these plasmas can in most cases be assumed to be under dynamical equilibrium: forces acting on the plasma body are balanced to zero. This is because the time scale of a dynamic response of a plasma will be ~ω p −1 or ω c −1 where ω p and ω c are plasma and cyclotron (angular) frequencies respectively. For the example of the solar wind these numbers are approximately 10−5 and 10−3 sec for electrons and 10−2 and 1 sec for protons.